A common formula/theorem when working with right triangles is called the Pythagorean Theorem.
Pythagorean Theorem: a^2 + b^2 = c^2
---a and b are legs of the triangle
---c is the hypotenuse
8^2 + 6^2 = c^2
64 + 36 = c^2
100 = c^2
c = 10
Answer: The length of the hypotenuse of the triangle is 10cm.
Hope this helps!